The inventors proposed such kind gap measurement device which uses a gap visualization method in patent documents 1 and 2.
Patent Document 1: JP-A 2010-156630
Patent Document 2: JP-A 2009-229312
In a gap visualization method, it is sometimes necessary to measure the gap distance at a specific point in a gap extending in a prescribed direction (an extended gap). In this case, a method may be used in which a mask provided with a pinhole, window, or the like, is arranged proximally to the gap and light from a gap portion other than the object point is blocked. Or, a method may be used in which a point light source is placed on the specific object point proximal to the gap so that light from a gap portion on the periphery is not produced.
However, with the former method which places the mask immediately after the gap, the light passing through the mask gets diffracted. One method for overcoming this drawback involves analyzing the diffraction pattern to learn the size of the gap, but there is a problem that the analysis is difficult. A situation also may arise in which it is difficult to place the mask proximally to the gap.
With the latter method which uses a point light source, when the location to be measured is small, it may be difficult to direct the light emitted from the light source to the target location. Also in the case of a thick measured object, a design for diagonally directing the light source, or the like, is necessary, and a spatial constraint arises.